An electric power steering apparatus (EPS) serves as an apparatus which is equipped with a motor control unit to control a motor. The electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of the motor, applies a driving force of the motor being controlled with an electric power supplied from an inverter to a steering shaft or a rack shaft by means of a transmission mechanism such as gears. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of duty command values of a pulse width modulation (PWM) control. A brushless motor, which has an excellent maintenance performance, is generally used as the motor.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, a handle shaft) 2 connected to a steering wheel (handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 are provided with a torque sensor 10 for detecting a steering torque Ts of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angle θ, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key (IG) signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist control (steering assist) based on a steering torque Ts detected by the torque sensor 10 and a vehicle speed Vs detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 for EPS based on a voltage control command value Vref obtained by performing compensation and so on with respect to the current command value.
As well, a steering angle sensor 14 is not indispensable and may not be provided. It is possible to obtain the steering angle from a rotational position sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 40 to receive/send various information of a vehicle is connected to the control unit 30, and it is possible to receive the vehicle speed Vs from the CAN 40. Further, it is also possible to connect a non-CAN 41 receiving/sending a communication, analog/digital signals, a radio wave or the like except with the CAN 40 to the control unit 30.
In such an electric power steering apparatus, the control unit 30 mainly comprises a micro controller unit (MCU) (including a CPU (Central Processing Unit), an MPU (Micro Processing Unit) and so on), and general functions performed by programs within the MCU are, for example, shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Ts detected by the torque sensor 10 and the vehicle speed Vs detected by the vehicle speed sensor 12 (or from the CAN 40) are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1, based on the steering torque Ts and the vehicle speed Vs with reference to an assist map or the like, which is a control target value of a current supplied to the motor 20. The calculated current command value Iref1 is inputted into a current limiting section 33 via an adding section 32A, the current command value Irefm that is limited the maximum current in the current limiting section 33, is inputted into a subtracting section 32B.
A deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im which is fed-back is calculated in the subtracting section 32B, and the deviation I is inputted into a proportional-integral (PI)-control section 35 for improving a current characteristic of the steering operation. The voltage control command value Vref that the characteristic is improved in the PI-control section 35, is inputted into a PWM-control section 36, and the motor 20 is PWM-driven through an inverter 37 serving as a driving section. The motor current value Im of the motor 20 is detected by a motor current detector 38 and is fed-back to the subtracting section 32B. An FET is used as a driving element in the inverter 37, and the inverter 37 comprises a bridge circuit of the FETs.
A compensation signal CM from a compensation signal generating section 34 is added at the adding section 32A. A characteristic compensation of the steering system is performed by adding the compensation signal CM, and then a convergence, an inertia property and so on are improved. The compensating section 34 adds a self-aligning torque (SAT) 34-3 with an inertia 34-2 at an adding section 34-4, further adds the result of addition performed at the adding section 34-4 with a convergence 34-1 at an adding section 34-5, and then outputs the result of addition performed at the adding section 34-5 as the compensation signal CM.
In such the electric power steering apparatus, clients (users) feel an uncomfortable steering feeling near a handle center and request for improvement thereof increases as a recent situation. A mechanical clearance near the handle center at a time of the vehicle driving, an existence of the dead band near the handle center as shown in FIG. 3 and an existence of inflection points C1 and C2 (rise-up of discontinuous assist torque) when the system deviates from the dead band are considered as one of causes of the above situation. In the conventional electric power steering apparatus, a fine control cannot be functioned with respect to the dead band near the handle center, and there is a possibility that a driver may feel uncomfortable.
For example, Japanese Unexamined Patent Publication No. 2007-237839 A (Patent Document 1) discloses the electric power steering apparatus which improves the steering feeling of the driver near the handle center. In Patent Document 1, the apparatus includes an actuator (motor) which can adjust the steering torque, calculates a target steering angle based on the steering torque, and controls an output torque of the actuator such that an actual steering angle is coincident with the target steering angle. In this case, when a direction of the steering torque varies by means of the steering of the driver, the target steering angle is set to a constant value or almost a constant value until a predetermined variation in the steering torque occurs.